Such a control unit is known, e.g. from DE 10 2008 050 835 A1 and DE 101 27 907 A1.
Hydraulic units and components thereof, such as hydrostatic pumps, hydrostatic motors, hydrostatic valves or power cylinders are often operated in explosion prone environments or in areas with risk of explosions. Therefore, they must be designed according to specific guidelines for the prevention of explosions. With respect to electrical devices this implies that the electrical power used in the controls are to be prevented to provoke an explosion, usually, for instance, by extensive and costly capsulation of the same requiring at the same time big installation space. In this case extensive protective measures are required such as explosion proof casings for the components. This permits the utilization of energies, high enough to ensure control forces sufficiently high for the actuation of valves or other hydraulic control devices. Transferring this to electrical proportional displacement controls, which are utilized frequently, e.g. in variable displacement pumps, this implies bulky casings for the isolation of proportional solenoids and of all related electrical contacts and power lines.
Alternatively, the electrical energy applied is reduced till that point at which possible ignition sparks are too weak to cause an explosion. This case is termed “intrinsically safe”. However, this alternative has a reduced energy level for generating control forces. Hence, such a solution of an explosion proof design has often the drawback that limited energies are often not sufficient to provide forces high enough for a reliable, high quality control/displacement of a hydraulic unit. Further, these relatively weak control forces are susceptible to superposition of external disturbances, further reducing the quality of control.
To adapt a common non-explosion-safe electrical proportional displacement control to an intrinsically safe design several aspects would have to be optimized. Spring forces would have to be reduced/adjusted to the level of available magnetic forces due to the low electrical energy usable to provide actuator forces. Furthermore, disturbances of any kind must be reduced and/or shielded sufficiently from the control unit in order to not generate disturbances. Reduction of mechanical friction and flow forces has to be done as well. Further, inertial forces would have to be reduced also, because they act with or against the magnetic forces, depending on the orientation of the displacement. All these efforts for such an approach with low electrical power would be substantial, however only marginally successful.
A different approach to achieve actuating forces sufficiently high for an intrinsically safe control unit design with small currents is e.g. the introduction of a hydraulic pilot or boost stage. Such a pilot or boost stage is described for example in DE 34 02 508. A hydraulic pilot valve within the control unit of a hydraulic machine with a variable displacement is also known from DE 10 2008 050 835 A1. Here, the proportional solenoids are relatively high powered and consequently not inherently safe. Further, the pilot stage pressure relief valve is situated remote from the main parts of the proportional adjustment, hence building a bulky system. Thus, the above mentioned prior art solutions have the draw-back that they are relatively bulky and/or not intrinsically safe.